fps option is missing in VideoMetadata

FPS option is missed in `class VideoMetadata(proto.Message)` in types/content.py

It should exist according to both docs (v1 and v1beta1):
https://cloud.google.com/vertex-ai/generative-ai/docs/reference/rest/v1/Content
https://cloud.google.com/vertex-ai/generative-ai/docs/reference/rest/v1beta1/Content

And also is existent in VideoMetadata in `content.proto`:
https://github.com/googleapis/googleapis/blob/97763d6efb2d35474e0363c254ee6994837a08fa/google/ai/generativelanguage/v1/content.proto#L107
as:
```
message VideoMetadata {
// Optional. The start offset of the video.
google.protobuf.Duration start_offset = 1
[(google.api.field_behavior) = OPTIONAL];

// Optional. The end offset of the video.
google.protobuf.Duration end_offset = 2
[(google.api.field_behavior) = OPTIONAL];

// Optional. The frame rate of the video sent to the model. If not specified,
// the default value will be 1.0. The fps range is (0.0, 24.0].
double fps = 3 [(google.api.field_behavior) = OPTIONAL];
}
```

打开 types/content.py 并检查 VideoMetadata 类，然后将其与链接的 content.proto 定义和 v1/v1beta1 文档进行比较。当 VideoMetadata 提供文档中所述的 fps 选项，且具有指定的默认值和范围，并与 proto 定义一致时，即表示完成。

由索引模型根据 Issue 内容生成。